Pyrotechnic actuator and method of actuating a pyrotechnic actuator

ABSTRACT

A pyrotechnic actuator and a method of actuating the same are described. The actuator comprises a pressure chamber and a piston having such an outer diameter that it is snugly fitted into the pressure chamber. A gas generating ignition and output charge generates gas that pressurizes the pressure chamber and the piston when ignited. The piston is designed such that at least part of its outer diameter expands under the pressure in the pressure chamber such that at least a part of the external piston surface is forced into a press fit against the internal wall of the pressure chamber and therefore seals the pressure chamber. This prevents exhaust gas and flames from escaping from the device in an uncontrolled manner.

FIELD OF THE INVENTION

The invention is in the field of controlling the release of primer/initiator exhaust gases and flames from pyrotechnic actuators to improve operational safety and to meet regulatory requirements. The invention is in particular in the field of pyrotechnic actuators such as pyrotechnic power disconnect devices, pyrotechnic battery disconnect devices, pyrotechnic fuses, pyrotechnic switches, pyrotechnic cable cutters, pyrotechnic release devices, pyrotechnic locking devices, and pyrotechnic interrupters.

BACKGROUND OF THE INVENTION

Pyrotechnics have been used in break, sever, disconnect or release devices and mechanisms for a long time. As an example, for instance applicant's U.S. Pat. Nos. 7,123,124 and 7,239,225 can be named, the content of which is herewith incorporated by reference. When some of these devices were actuated, the primer exhaust gases and/or flames were released to the surroundings. In some applications the release of hot gases, smoke and/or flames may not be a problem. However, in many applications the release of hot gases and/or flames during device functioning may create a safety hazard for equipment and/or personnel near such devices. Devices that do not control the release of hot gases or flames are also considered hazardous by regulatory agencies in US and worldwide and require special packaging, storage and regulatory permits for transportation and the use of the product throughout the life of the product. The requirements are stringent enough that they may prevent the use of such devices in certain applications especially some cost sensitive applications due to the additional costs involved. It will also prevent the product from being distributed as ordinary consumer product such as automotive parts and keep the general public and industry from the benefits of using such devices.

Certain device configurations are more conducive to sealing the exhaust gases and flames than others. Devices primarily made from metals in tubular shapes using a piston, cylinder and O-rings (such as Reefing Line Cutters) tend to be relatively easy to seal as to gases and flames during the device functioning and have been known art for a long time. It is not practical to make all such devices out of metals such as in this case of electrical power/battery disconnects or to have a tubular piston-cylinder with O-ring configuration. Some applications require odd shapes for device configuration that are much more difficult to seal than simple O-ring type designs. Low cost applications such as automotive or consumer products require minimizing the costs of sealing apparatuses (gaskets, O-rings, sealants). It is sometimes impossible or not practical to verify if the seal is in place and installed correctly once the device is closed. For parts that are expected to be used over many years of service the useful life of gaskets, O-ring or sealant is also a limiting factor especially in harsh installation and operating environments.

SUMMARY OF THE INVENTION

It is an object of the invention to alleviate or remove entirely the hazards created by firing pyrotechnic actuators, particularly the hazards created by flames and/or exhaust gases escaping from the device after firing.

According to a first aspect of the invention, this is achieved by a pyrotechnic actuator comprising: a pressure chamber that is defined by an internal pressure chamber wall and an outer piston wall; a piston having such an outer diameter that it is snugly fitted into the pressure chamber such that at least a part of an external piston surface extends in close proximity to or abuts against at least a part of the internal pressure chamber wall; a gas generating ignition and output charge generating gas that pressurizes the pressure chamber and the piston when ignited; and an igniter for igniting the gas generating ignition and output charge; wherein the piston is designed such that at least part of its outer diameter expands under the pressure in the pressure chamber such that at least a part of the external piston surface is forced into a press fit against the internal wall of the pressure chamber.

According to a second aspect of the invention, this is achieved by a method of actuating a pyrotechnic actuator that comprises a pressure chamber that is defined by an internal wall and a piston having such an outer diameter that the piston is fitted snugly into the internal wall of the pressure chamber such that at least a part of an external piston surface extends in close proximity to or abuts against at least a part of the internal wall surface; the method comprising: igniting a gas generating charge of an explosive; pressurizing the pressure chamber and at least one surface of the piston with the gas generated by the ignited charge of an explosive; expanding the piston in its outer diameter by pressure within the pressurized pressure chamber such that at least a part of the external piston surface is forced into a press-fit against the internal wall of the pressure chamber; and propelling the piston by the pressure in the pressure chamber along the same while maintaining the press fit created by the pressure induced piston expansion.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the pyrotechnic actuator, the piston is at least in part hollow comprising an internal piston wall that forms a part of a piston surface defining the pressure chamber together with the internal pressure chamber wall. The hollow design can be of any kind as long as the hollow space formed within the piston is connected to the pressure generated by the gas after firing.

Preferably, the internal piston wall is formed as a depression in a rear end face of the piston. This is particularly easy to manufacture. However, it may be desirable to have the pressure access a larger hollow space via a smaller channel so that the pressure inside the piston is built in a controlled manner more slowly, allowing the piston to be propelled easier along the pressure chamber while sealing action gets stronger when the piston comes closer to reaching its end position.

According to another preferred embodiment of the pyrotechnic actuator the depression has such a shape and size that it determines which part of the piston expands in its outer diameter and to what extent it expands. This is a valuable design feature for controlling the sealing forces and what parts of the piston should generate the sealing forces. One such preferred shape is for instance to have the depression start from the rear end face as a cylindrical depression that is followed towards a front end face of the piston by a conical section narrowing towards the front end face. In this configuration, the rear end of the piston expands more than the front end. Preferably, it would for instance be possible to provide an additional sealing member like an O-ring at the front part of the piston where the depression is narrow and therefore where the piston expands less so that in addition to the pressure induced sealing action by piston expansion also some independent sealing irrespective of any gas pressure induced sealing action is provided.

Another preferred shape of the depression is to have it start from the rear end face as a cylindrical depression that is followed towards a front end face of the piston by a semi-spherical section narrowing towards the front end face. The semi-spherical shape prevents stress peaks at the outer circumference of the bottom of the depression where the bottom connects to the cylindrical depression wall since the spherical bottom shape prevents a sharp annular corner between the bottom and the cylindrical wall of the piston.

According to another preferred shape of the depression it starts from the rear end face as a cylindrical depression that comprises a dome-shaped bottom that deepens from an intersection with a piston center line towards an outer circumference of the bottom. The connection between the dome-shaped bottom and the cylindrical wall of the depression can be rounded for avoiding stress peaks. This shape provides the advantage of facilitating sort of an annular joint that allows the piston wall to be deflected more easily to the outside, i.e. facilitates letting the piston expand where the depression is provided.

According to another preferred embodiment, the depression in a rear end face of the piston comprises an inner diameter that is at least half of the outer diameter of the piston and extends over at least one third of a length of the piston. As already discussed above, the inner diameter of the depression and the outer diameter of the piston determine the wall thickness as one parameter determining how big a force the piston exerts on the pressure chamber wall. Another parameter is the length of the depression from the rear towards the front of the piston, and therefore over what length the piston may expand in its diameter. In most applications, it will not be necessary to have the depression extend over almost the entire length of the piston since the front end of the piston may extend outside the pressure chamber after the piston has reached its end position. Therefore, a pressure induced sealing action might only be desirable closer to the rear end of the piston.

According to another preferred embodiment of the pyrotechnic actuator the gas generating ignition and output charge is provided in an output can that is ignited by electrical contacts and the output can is accommodated in the depression before the ignition and output charge is ignited. This has the advantage of reducing the entire length and in addition of accommodating the output can with its explosive securely within the piston as an additional protection. However, it would also be possible to accommodate the output charge and igniter at any other position as long as it is somehow connected to the pressure chamber and can transmit the pressurized gas into the pressure chamber after firing.

According to another preferred embodiment of the pyrotechnic actuator the external piston surface comprises a plurality of semicircular ridges which are press-fitted into the internal pressure chamber wall. The advantage of this design is to hold the piston in place by the press-fit, but at the same time prevent too strong a press-fit for allowing the piston to move easily along the pressure chamber after firing. While such a press-fit would also be possible without the semicircular ridges or other shapes such as barbs, these help particularly to control the press-fit forces especially when exposed to extreme temperature swings in the field and provide better sealing capability over wider temperatures.

According to another preferred embodiment of the pyrotechnic actuator a stop limiting the maximum movement of the piston along the pressure chamber is provided such that the press-fit between the external piston surface and the internal wall of the pressure chamber is maintained. The stop allows the piston to be propelled forward only up to the point where the rear end is not released from the pressure chamber and therefore may still provide sufficient sealing. This feature positively wedges/locks the piston in between the severed ends of the conductor increasing disconnect reliability.

According to another preferred embodiment of the pyrotechnic actuator the press-fit between the external piston surface and the internal wall of the pressure chamber substantially seals the pressure chamber retaining the generated gas within the pressure chamber. Substantially sealing is understood that it takes a long time until the pressure is released. A hermetical, 100% sealing is practically impossible as pressurized gas diffuses, in particular if a plastic is used as the material for the housing and the pressure chamber and/or piston, but even if metal is used. Also, though the pressure-induced sealing forces provide a relatively good sealing, gas may nevertheless over time gradually diffuse or leak between the contact surfaces of the pressure chamber and the external piston surface. However, a release of the pressure over time is not only acceptable but even desirable in most environments for safety reasons. In the alternative, it is also possible to provide an extra valve allowing releasing the pressure when desired. The gas can also be released under these controlled circumstances through a filter or a reaction column adsorbing or chemically altering the exhaust gas.

According to another preferred embodiment of the pyrotechnic actuator the pyrotechnic actuator is a part of one of the group consisting of: pyrotechnic power disconnect devices, pyrotechnic battery disconnect devices, pyrotechnic fuses, pyrotechnic switches, pyrotechnic cable cutters, pyrotechnic release devices, pyrotechnic locking devices, pyrotechnic interrupters, and pyrotechnic dislodging devices. The list of possible applications for the pyrotechnic actuator is not to be understood as limited by the above applications. The actuator can be used for any application where a reliable, fast and strong action is required that can be performed by an explosive propelling a piston forward. The actuating parameters such as speed and force can be determined by the type and quantity of the used charge of explosive.

According to another preferred embodiment of the pyrotechnic actuator the front end face of the piston is connected to at least one of a latch and a knife. However, many other alternatives are possible, for instance by a planar front end face capable of either pushing any target element aside or capable of shearing off any element over a shearing edge such as for instance an electrical conductor that should be interrupted. Other alternatives might be a pointed end for puncturing an element or container as desired. Accordingly, preferred actions performed by the method of actuating a pyrotechnic actuator are cutting, latching, releasing, puncturing, coining, shearing etc.

According to another preferred embodiment of the method of actuating a pyrotechnic actuator the piston is stopped at a maximum traveling range by running the piston against a stop. This stopping action has already been described above. There might be many reasons why the maximum traveling range of the piston should be limited, for instance for holding the rear end of the piston within the pressure chamber and therefore maintaining a tight sealing.

According to another preferred embodiment of the method of actuating a pyrotechnic actuator the pressure chamber may be imperfectly sealed by expanding the piston into the press-fit such that a gradual releasing of the pressure within the pressure chamber takes place over time. As discussed above, the imperfect sealing might be desired so that the pressure is automatically released within a reasonable time, but so slowly that it does not create a hazard. A controlled release serves the purpose of removing the pressure and therefore avoids any dangers that might otherwise result from a highly pressured chamber, for instance if parts fail or an unauthorized dissembling is carried out, for instance by maintenance personnel.

The invention achieves a method of sealing exhaust gases and flames from uncontrolled release to the surroundings in tubular and/or odd shaped actuator devices without the use of additional components such as gaskets, O-ring or sealants with high reliability and for mistake-proofing the device assembly. The design and the materials used in the construction of the piston, actuator, thruster, pusher is configured such that it also acts as a sealing apparatus inside the pressure cavity during device functioning and prevents the uncontrolled release of the exhaust gases and/or flames to the surrounding. The design is flexible enough to accommodate a variety of applications involving a range of conductors, strips, cables, lines, ropes, etc. that can be severed, disconnected, displaced, dislodged or actuated, captured, engaged or locked in place.

The actuator may be cylindrical in shape or may have more than two sides to it. It may have a flat actuation/cutting end or may have application specific features to facilitate orientation, force concentration, guillotine cutting edge or ability to lock in place. The space for primer inside the piston/actuator may be cylindrical or have some other desirable shape. The individual piston/actuator could be any combination of features specified here depending on application. The pressure cavity may be configured to accommodate the piston as required by the application. A number of materials may be suitable for making the actuator/piston. This includes but is not limited to Hot Melts, a variety of fiber filled and unfilled thermoplastics (Polyethylene, Polypropylene, Polyurethane, Polyamides, Polyimide, Polyester, PEEK, PPS, LCP and many more) or thermoset polymers (natural and artificial rubber/elastomers), single or multiple part epoxies/sealants or metals and alloys. Various types of ceramics may also be used. The piston may have a hexagonal shape. This shape and similar alignment features (i.e. tongue and groove, square) will prevent the actuator from rotating during deployment and direct the guillotine-shaped knife or locking feature exactly at the location where it is needed. As another alternative, a cylindrical shaped actuator can be used. Ridges having a semicircular profile can be provided on the outer diameter of the actuator or piston and pressed fit to the inside diameter of the pressure cavity providing excellent pressure sealing capability without the use of O-rings. The cutting face may or may not have a guillotine-shaped knife or may use a locking feature depending on application.

As another alternative, a rectangular actuator with a locking/capturing element can be used. The outer perimeter is closely matched with that of the pressure cavity with little clearance between the two. When the device is fired the actuator may move and lock itself into a slot on the mating part.

According to still another alternative, a cylindrical actuator with an option to install an O-ring can be used. The cutting face may be flat or have a guillotine-shaped knife, or a locking feature as required by the application. The outer diameter of the actuator is closely matched with the inner diameter of the pressure cavity. The optional O-ring provides a double redundant sealing system to increase reliability.

The primer cavity inside the actuator has multiple purposes. The primer sits inside the cavity to reduce the overall length of the assembly. Once the primer is fired, the thinner actuator walls surrounding the primer expand due to primer pressure and seal the pressure which in turn generates the force or thrust to do work. Once the actuator has fully stroked, enough of the piston is left in the pressure chamber to prevent any uncontrolled leaks around the actuator. The actuator is designed to be tight fitting inside the pressure chamber and when it is fully stroked it will deform and swell up due to firing energy and heat and will not retract back into pressure chamber under service loads. This locking feature is desirable in applications such as power/battery disconnects to prevent reconnect due shock or vibration.

The component parts of the pyrotechnic actuator including its housing, the pressure chamber and the piston may be molded, machined, forged, stamped or deep drawn as required by application.

If non-cylindrical type actuators are used, the cutting edge or locking or dislodging feature can be oriented with high reliability as required by the application. A preferred concept as to the manufacturing costs and the flexibility in possible shapes and modifications is a plastic housing concept. However, metallic housing of circular or irregular shapes may also be used by applying the aforementioned concepts. The ideas are not limited to disconnect devices but can be adapted to any product that requires an actuator to move for serving a specific function.

The design can be made mistake proof during manufacturing and installation. This can be done by using symmetrical parts such that flipping around parts that will not impact form, fit or functionality of the device is possible. In areas where part symmetry was not possible or practical, parts are designed such that it is impossible to assemble the device if the part orientation is not correct.

In the prior art the sealing was primarily achieved with the use of O-rings, gaskets and sealant. In sealed devices like these, it is very difficult if not impossible to inspect for presence of an O-ring or gaskets after the assembly is completed. Not using O-rings or gaskets according to the present invention increased the reliability of the device.

The bottom housing may carry according to a preferred embodiment a shearing edge that acts as a secondary barricade to prevent the uncontrolled release of primer gases and flames to the surroundings which could start fires causing equipment damage and/or harm to personnel in close proximity of the device. This is double redundant safety. Sealing the primer exhaust with the actuator opens up the possibility of using a variety of conductor strips without compromising reliability or limiting the number of applications.

The piston can preferably only be installed one way such that if it is reversed it will not fit inside the pressure cavity and the assembly cannot be completed. The piston's cutting edge preferably always faces the strip to be severed in this embodiment. The conductor strip can only be installed one way such that the width that needs to be severed is always lined up with the shearing edge of the piston and housing. This is done with unique features in the molded housing and conductor. Putting the conductor upside will not change form, fit or function. The top housing with pressure chamber can be rotated 180° without changing form, fit and function. The firing reliability of the device is independent of primer pin polarity or lead wire orientation. The fully assembled device can be installed in the field in any orientation without impacting form, fit, functionality or reliability of the device.

The aforementioned concepts can be easily adapted to various sizes and configurations depending on application. Plastic or metallic housings (or a combination of either) of circular, square or irregular shapes may also be used using these concepts.

The present invention provides reliable and cost effective mistake proof solutions to address the aforementioned requirements and overall device safety.

In the following, the present invention and its advantages and equivalents are discussed in more detail by describing exemplary embodiments implementing the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a pre-firing sectional view of a preferred device implementing the pyrotechnic actuator according to the present invention.

FIG. 2 shows a post-firing sectional view of the same device as shown in FIG. 1.

FIG. 3 shows a sectional view and a rear end view of a first embodiment of a piston forming a part of the pyrotechnic actuator according to the present invention.

FIG. 4 shows a sectional view and a rear end view of a second embodiment of a piston forming a part of the pyrotechnic actuator according to the present invention.

FIG. 5 shows a sectional view and a rear end view of a third embodiment of a piston forming a part of the pyrotechnic actuator according to the present invention.

FIG. 6 shows a sectional view and a rear end view of a fourth embodiment of a piston forming a part of the pyrotechnic actuator according to the present invention.

FIG. 7 shows a perspective view of a fifth embodiment of a piston forming a part of the pyrotechnic actuator according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional view of one embodiment of the present invention in the pre-firing state. The shown embodiment demonstrates a pyrotechnic power disconnect device. A housing 1 comprises an upper housing part 2 and a lower housing part 3 which can, during assembly, be joined along a separation line 4. During assembly, an electrical conductor 5 can be inserted on top of the lower housing part 3. In parallel, the upper housing part can be pre-assembled by inserting the piston 6 and an output can 7 comprising according to this embodiment an ignition and output charge that can be ignited via electrical contacts 8. In the pre-firing state, the output can 7 is accommodated in a depression 9 that is formed in a rear end face 10 of the piston 6. For the purpose of pre-assembling the upper housing part 2, the piston 6 can be press-fitted into an internal pressure chamber wall 11. Only a relatively light press-fit is necessary, just for holding the piston in place within the internal pressure chamber wall. For the purpose of creating a controlled press-fit, according to the shown embodiments, a plurality of semicircular ridges 15 are provided on the external piston surface 16, meaning that these do of course extend around the entire circumference of the piston but have a semicircular or similar shape, for instance the profile of a segment of a circle. In the alternative, the piston 6 can be held in place by means of a small bead of glue or any other suitable means.

After pre-assembly, the upper housing part 2 can then be positioned on top of the lower housing part 3, sandwiching the electrical conductor 5 between the upper and lower housing parts 2 and 3. Both housing parts 2, 3 can then be fastened together by rivets 12. These rivets 12 and their swaged end 13 are selected such that these have sufficient strength in order to withstand the force from the piston 6 when propelled forward in direction of the lower housing part 3 after firing the output charge. The rivets make the device temper resistant and difficult to disassemble and access the small explosive charge inside. In an alternate embodiment the housing parts 2, 3 can be bolted together using plastic, wooden, sheet metal or machined screws. At least the upper housing part 2 is preferably made from plastic. Preferably both the upper housing part 2 and the lower housing part 3 are made from plastic. For providing a sufficient reaction force to keep the upper and lower housing parts together after firing, it is also possible to make the lower housing part from a metal in order to provide stronger threads 13. In the alternative to rivets, it is also possible to provide a metal bolt screwed into a metal nut that can be tightened against a bottom 14. For preventing unauthorized disassembly, glue can be provided on the threads immediately prior to screwing the bolt and the nut together, or a weld bead can be placed to prohibit unauthorized unscrewing.

FIG. 2 shows the same sectional view as FIG. 1, but in the post-firing state. The same elements as shown in FIG. 2 are provided with the same reference numerals, but not all reference numerals are repeated, only those elements that are discussed in the following. After igniting the ignition and output charge provided in the output can 7, the output can 7 explodes, releasing the generated gas into the pressure chamber 17. As it becomes apparent from FIG. 2, the piston 6 has been propelled forward, i.e. in the drawing downward and has ruptured the electrical conductor 5. For facilitating this rupturing action provided by the piston 6 that has been propelled forward, a shearing edge 18 is provided. An internal bottom 19 of a cavity 20 accommodating a substantial partial length of the piston 6 in the post-firing state effectively forms a stop that limits the maximum travelling range along which the piston 6 can be propelled forward. This internal bottom 19 also provides the additional function of a counter pressure surface against which of the ruptured conductor 5 is pressed by a front end face 21 of the piston 6. This provides another safety feature for preventing that the two parts of the now ruptured electrical conductor 5 can inadvertently reconnect and re-establish an electrical contact that has on purpose been interrupted in the post-firing state.

It becomes also apparent from FIG. 2 that the depression 9 that is formed in the rear end face 10 of the piston 6 is part of the pressure chamber 17 and is therefore filled with pressurized gas in the post-firing state. This pressure expands the outer diameter of the piston 6 so that the external piston surface 16 is pressed against the internal pressure chamber wall 11 and establishes a sealing that retains the pressurized gas within the pressure chamber 17. Depending on the circumstances, it might be desirable to contain the pressurized gas within the pressure chamber 17 for some period of time, or in the alternative, it might be acceptable if the pressurized gas gradually releases by creeping or diffusing through the press-fit between the external piston surface 16 and the internal pressure chamber wall 11.

The sealing quality and sealing force between the external piston surface 16 and the internal pressure chamber wall 11 depends on several factors, for instance on the material that is chosen for the piston 16 and the size of the depression 9, and therefore by the thickness of a wall 22 which thickness is determined by a diameter difference between an inner diameter of the depression 9 and an outer diameter of the piston 6.

FIGS. 3 to 7 demonstrate different shapes and designs of various pistons, depending on the various utilities. Turning to FIG. 3, the forward end of the piston 6 is provided with a guillotine-like knife edge 23 for cutting a cable. This knife edge 23 transitions into a hexagonal cross section of the piston 6 having an external diameter 24, while the depression 9 has a cylindrical cross-section of an inner diameter 25. In this connection, for the purpose of this patent application, the expression “diameter” is understood as the widest measurement of a cross section and is not restricted to circular cross-sections. The partial figure on top of FIG. 3 shows the rear end face 10 of the piston 6 as viewed when the knife edge 23 in the drawing below is moved into the drawing plane by 90 degrees while the rear end face 10 is moved out of the drawing plane by 90 degrees. Similar combinations of piston sectional views in combination with piston rear end views as shown in FIG. 3 are also shown in FIGS. 4-6. In the embodiment shown in FIG. 3, the depression 9 comprises a cone-shaped bottom 26 that is deeper in its outer circumference than in the middle. This shape may help expanding the outer diameter 24 of the piston 6 when the depression 9 is pressurized by gas.

The piston shown in FIG. 4 has already been described in the embodiments shown in FIGS. 1 and 2. The same reference numerals are used as in the remaining figures. It is pointed out that the depression 9 has a square shape, while the cross-section of the piston 6 has a circular shape. Semicircular ridges 15 are provided on a front portion 27 of the piston 6, while a rear portion 28 of the piston 6 is substantially cylindrical. This cylindrical part is the part that is widened in its diameter 24 and therefore pressed against the internal wall 11 of the pressure chamber (see FIGS. 1 and 2) when filled with pressurized gas. The front face 21 of the piston 6 is flat since it has only the function of shearing the electrical conductor 5 (FIG. 2) by pressing it against the shearing edge 18 (FIG. 2).

The piston shown in FIG. 5 has a substantially rectangular shape that transitions into a male latching portion 29 that likewise has a rectangular shape, but much smaller than the cross section of a rear part 31 of the piston 6. The depression 9 comprises a spherical bottom 30. When the piston is pushed forward, the male latching portion 29 may engage a respective female latching portion.

FIG. 6 shows another variance of the piston 6 wherein the depression 9 comprises a rear portion 32 and a front portion 33. While the rear portion 32 is cylindrical, the front portion 33 is conical having the deepest spot of this front portion 33 where a center line of the entire piston intersects with the depression 9. In addition, an annular groove 34 can be provided for holding an O-ring providing additional sealing. As can be seen, the thickness 35 of a wall portion at the rear of the piston 6 is relatively small in comparison to the wall thickness at the front portion 33. Since the O-ring is provided at the front, less expansion is needed at the front of the piston in comparison to the rear for providing a sealing under pressure. The shape of the depression takes this into account. Therefore, the shape of the depression 9 can be adapted to control which part of the piston 6 should expand the most.

FIG. 7 shows a perspective view of a piston 6 similar to FIG. 4, but comprising a depression 9 that has a circular cross section, and in contrast to the embodiments shown in FIG. 4, the semicircular ridges 15 are provided over the entire length of the piston 6 and not just in the front part.

All embodiments shown and discussed are just exemplary embodiments and the invention is to be understood as not being limited to these exemplary embodiments. Many modifications are possible and within the scope of the present invention. 

1. A pyrotechnic actuator comprising: a pressure chamber that is defined by an internal pressure chamber wall and an outer piston wall; a piston having such an outer diameter that it is snugly fitted into the pressure chamber such that at least a part of an external piston surface extends in close proximity to or abuts against at least a part of the internal pressure chamber wall; a gas generating ignition and output charge generating gas that pressurizes the pressure chamber and the piston when ignited; and an igniter for igniting the gas generating ignition and output charge; wherein the piston is designed such that at least part of its outer diameter expands under the pressure in the pressure chamber such that at least a part of the external piston surface is forced into a press fit against the internal wall of the pressure chamber.
 2. The pyrotechnic actuator of claim 1, wherein the piston is at least in part hollow comprising an internal piston wall that forms a part of a piston surface defining the pressure chamber together with the internal pressure chamber wall.
 3. The pyrotechnic actuator of claim 1, wherein the internal piston wall is formed as a depression in a rear end face of the piston.
 4. The pyrotechnic actuator of claim 3, wherein the depression has such a shape and size that it determines which part of the piston expands in its outer diameter and to what extent it expands.
 5. The pyrotechnic actuator of claim 4, wherein the depression starts from the rear end face as a cylindrical depression that is followed towards a front end face of the piston by a conical section narrowing towards the front end face.
 6. The pyrotechnic actuator of claim 4, wherein the depression starts from the rear end face as a cylindrical depression that is followed towards a front end face of the piston by a semi-spherical section narrowing towards the front end face.
 7. The pyrotechnic actuator of claim 4, wherein the depression starts from the rear end face as a cylindrical depression that comprises a dome-shaped bottom that deepens from an intersection with a piston center line towards an outer circumference of the bottom.
 8. The pyrotechnic actuator of claim 3, wherein the depression in the rear end face of the piston comprises an inner diameter that is at least half of the outer diameter of the piston and extends over at least one third of a length of the piston.
 9. The pyrotechnic actuator of claim 3, wherein the gas generating ignition and output charge is provided in an output can that is ignited by electrical contacts and the output can is accommodated in the depression before the ignition and output charge is ignited.
 10. The pyrotechnic actuator of claim 1, wherein the external piston surface comprises a plurality of semicircular ridges which are press-fitted into the internal pressure chamber wall.
 11. The pyrotechnic actuator of claim 1, further comprising a stop limiting the maximum movement of the piston along the pressure chamber such that the press-fit between the external piston surface and the internal wall of the pressure chamber is maintained.
 12. The pyrotechnic actuator of claim 1, wherein the press-fit between the external piston surface and the internal wall of the pressure chamber substantially seals the pressure chamber retaining the generated gas within the pressure chamber.
 13. The pyrotechnic actuator of claim 1, wherein the pyrotechnic actuator is a part of one of the group consisting of: pyrotechnic power disconnect devices, pyrotechnic battery disconnect devices, pyrotechnic fuses, pyrotechnic switches, pyrotechnic cable cutters, pyrotechnic release devices, pyrotechnic locking devices, pyrotechnic interrupters, and pyrotechnic dislodging devices.
 14. The pyrotechnic actuator of claim 1, wherein a front end face of the piston is connected to at least one of a latch and a knife.
 15. A method of actuating a pyrotechnic actuator that comprises a pressure chamber that is defined by an internal wall and a piston having such an outer diameter that the piston is fitted snugly into the internal wall of the pressure chamber such that at least a part of an external piston surface extends in close proximity to or abuts against at least a part of the internal wall surface; the method comprising: igniting a gas generating charge of an explosive; pressurizing the pressure chamber and at least one surface of the piston with the gas generated by the ignited charge of an explosive; expanding the piston in its outer diameter by pressure within the pressurized pressure chamber such that at least a part of the external piston surface is forced into a press-fit against the internal wall of the pressure chamber; and propelling the piston by the pressure in the pressure chamber along the same while maintaining the press fit created by the pressure induced piston expansion.
 16. The method of claim 15, further comprising stopping the piston at a maximum traveling range by running the piston against a stop.
 17. The method of claim 15, further comprising at least one of cutting, latching and releasing by the moving piston.
 18. The method of claim 15, further comprising substantially sealing the pressure chamber by expanding the piston into the press-fit.
 19. The method of claim 15, further comprising imperfectly sealing the pressure chamber by expanding the piston into the press-fit such that a gradual releasing of the pressure within the pressure chamber takes at least 1 minute. 